1. Field of the Invention
The invention is generally concerned with power transmissions, particularly continuously variable transmissions, and more specifically transmissions for use in rotational machines requiring continuous, infinitely adjustable output speed and output torque while maintaining nearly constant rotational speed of the input prime mover.
2. Background Art
A continuously variable transmission suitable for automotive applications has been sought for nearly a century. It has even longer been recognized that pedal-driven vehicles would operate most efficiently if the propulsive ground wheel rotational speed were varied such that the input pedal rotational speed remains nearly constant, independent of the incline of the path. Tools designed to remove material in a manufacturing process (lathes, drills, mills, routers, and the like) often benefit from precise selection of the tool speed at the interface with the workpiece. For cost and power efficiency reasons, such tools typically are powered by synchronous electric motors. Both machine power sources (such as electric motors or internal combustion engines) and human propulsion operate most efficiently at fixed rotational speeds or within a limited range; however, the final application of the driving power usually requires a different or broader range of speeds. For all applications, whether for machine powered equipment or human powered vehicles, a transmission device combining the desirable characteristics of high torque capacity, high efficiency, compact size, light weight and competitive manufacturing cost has yet to be achieved.
3. Background Art
Currently, speed adjustment is normally accomplished by the use of control devices incorporating numerous selective discrete fixed ratio elements (usually gears).
Continuously variable speed control systems (transmissions) are an alternative means for speed adjustment, but tend to occupy large volumes, are heavy, often use some sort of belt system to adjust the speed, or use complicated ratchet and overriding clutch mechanisms. Most known continuous speed control systems offer the capability of producing adjustable speed in only one direction and require a clutch to uncouple the prime mover from the output. Generally, all known variable speed control systems have limited power transfer capabilities.
High power applications, usually involving a limited range of speed variation, such as in construction and agricultural equipment, are currently obtained using hydrostatic drives operating in low flow, high fluid pressure regimes, or hydraulic torque converters operating in high fluid flow, low pressure conditions, or limited slip differential transmissions, all of which suffer significant energy loss. Continuously variable transmissions have yet to be suitably integrated into high-speed/high-power applications such as standard motor vehicles. Continuously variable transmissions to date have yet to exceed the approximately 150 hp rating.
Infinitely Variable Versus Continuously Variable Transmission
Several devices have been proposed for achieving continuously variable output speed, some of which include infinitely variation capability. A xe2x80x9ccontinuously variablexe2x80x9d transmission is a transmission in which the ratio of output rotation speed to input rotation speed can be varied continuously from a first value to a second value, both values having the same algebraic sign. A continuously variable transmission may also include a discrete, usually separately actuated, reverse gearxe2x80x94having an algebraic sign different from the first and second value. An xe2x80x9cinfinitely variablexe2x80x9d transmission is a transmission in which the ratio of output rotation speed to input rotation speed can be varied continuously from a first value to a second valuexe2x80x94where the first and second values can have different algebraic signs. Thus, the xe2x80x9cinfinitely variablexe2x80x9d transmission includes the xe2x80x9cinfinitexe2x80x9d condition where the ratio of the input rotation speed to the output rotation speed is indeterminate, i.e., infinite. Thus xe2x80x9cinfinitely variablexe2x80x9d transmissions may be characterized as a subset class of xe2x80x9ccontinuously variablexe2x80x9d transmissions, in that both classes have the capability to continuously control output speeds with generally fixed input rotational speed. However, infinitely variable transmissions offer a broader range of capability and applications due to their ability to drive output speeds to nearly zero while theoretically producing output torques approaching an infinite condition, limited only by the slip or load carrying capabilities of internal components.
Classes of Continuously/Infinitely Variable Transmissions
Both continuously variable and infinitely variable transmissions can be classified into five types.
The first type, which is the oldest and probably most extensively employed, includes two variable pitch pulleys connected by a belt with provision for varying the diameters of the pulleys and thus the speed ratio. While such devices are efficient, they characteristically are high in volume and weight, and have a limited range of speed variation. There have been at least two major improved variants to this basic dual variable pitch pulley concept. The power-limiting component in this design typically is either the belt reaching the limit of its tensile strength, or the friction between the belt and the pulley of smaller diameter. In a device disclosed in U.S. Pat. No. 3,720,113 to Van Dorne, the belt is changed from transferring torque via tension to transferring torque via compression. In the Van Dorne device, compression links are carried by a series of thin bands, the links conforming to each other to form a semi-rigid bar between the two variable pitch pulleys. The failure mode for the endless belt is changed from a tensile failure to one of buckling instability of the links, or material compression failure, both of which potentially allow a greater load than can be achieved by a tensile member. However, the speed adjustment range is limited.
A second major variant has been termed the Positive Infinitely Variable (PIV) variable speed drive used routinely in industrial applications. Within the definition used in this disclosure, the PIV is a misnomer because the speed control devices do not have a speed range where the output can be continuously varied to a negative algebraic sign. A feature of the PIV is the replacement of the belt by an endless chain, each link of the chain containing a series of transverse conforming rods that engage the edges of pulleys containing radial groves in the contact faces. This design eliminates slip between the endless belt-like member that transfers the torque and the mating variable pulley. While a speed variation range of as high as 6 is reported for such devices, the power ratings typically are below 30 hp.
A second type of continuously variable transmission includes single contact traction or friction drives using various schemes which rely on metal-to-metal rolling contact friction, sometimes using lubricant shear as the traction mechanism. Examples of such devices include cone on cone devices wherein two cones each of equal and opposite pitch are mated to contact at single points but in such a way that the summed circumference of the combined assembly is constant. Examples and variants of this type are shown in U.S. Pat. No. 4,392,394 to Hofbauer and U.S. Pat. No. 5,433,675 to Kraus.
Another example of continuously variable transmission is the ball and disc type. In this class of drive mechanism, the rotational axis for a ball element is usually at a substantially right angle to the rotational axis of a disc element. The ball element, which is constrained to have a surface of rotation, is positioned so that when it is pressed against the rotating disc element, the ball element is driven by the disc. By moving the ball element along a radius of the disc element, a variable speed drive can be obtained from the ball element. The efficiency of such mechanisms is highly dependent on the quality of contact between the two traction elements, the cleanness of the surfaces, and degree of wear of each of the elements. While substantial speed variation can be achieved, such devices a typically limited to sub-horsepower ratings, and are difficult to maintain.
The third type of continuously adjustable speed devices are hydraulic drives, typically driving hydraulic motors using variable displacement pumps. Such devices are termed hydrostatic drives because they operate at high fluid pressures, but with low displacement. Other variable speed hydraulic drives combine both gear sets and the hydrodrive mechanism to allow for infinitely variable capability. Examples are disclosed in the U.S. Pat. No. 5,624,015 to Johnson and U.S. Pat. No. 5,396,768 to Zulu. Such devices have proven reliable in high power and high torque applications, but at the cost of very low efficiency.
The fourth type of continuously variable speed control is in the general category of ratcheting drives. Such drives, as taught for example by Pires (U.S. Pat. No. 5,1334,115), Gogins (U.S. Pat. Nos. 4,116,083, 4,194,417, 4,333,555, 4,936,155, and 5,392,664), and Mills (U.S. Pat. No. 5,632,702) are all characterized as completely mechanical and function by generating variable amplitude oscillation by positioning control rings or cams eccentrically to the input drive. Generally a plurality of eccentric positions are used for a single revolution of the input shaft with connecting arms transferring forward portion of the oscillating motion generated in each arm to an output gear through over riding clutches, or other mechanical diode devices. To reduce output torque ripple, such devices typically use an increasing number of oscillating elements, and auxiliary linkages to smooth the rectified motion to an acceptable level for a given application.
The inventions described and claimed herein fall generally within a broad, fifth type of continuously variable speed control mechanism generally characterized by the use of one or more epicyclical gear arrangements. Some known devices of this type contain elements of both the ratcheting drive and epicyclical control, for example the device of U.S. Pat. No. 5,334,115 to Pires. The common element of control in this type of transmission is the unique motions associated with epicyclic systems. One significant distinction between the present invention and various known versions of this category of transmissions is the manner in which the output motion is connected to the input, and the means by which rotary force is imparted to each of the epicyclic components. Also of note in any particular version is whether and how the planetary carrier (spider), the planetary assemblies, the encircling ring gear, the sun gear, or auxiliary xe2x80x9cmoonxe2x80x9d gears that attach and circulate about a portion of the circumference of planetary assemblies, interact. Many of the known devices rely on some type of friction device to create a change of speed that is then amplified, or smoothed by the epicyclic system.
By way of example, the following United States Patents describe various epicyclical type systems: U.S. Pat. No. 4,567,789 to Wilkes; U.S. Pat. No. 5,632,703 to Wilkes et al.; U.S. Pat. No. 1,445,741 to Blackwell; U.S. Pat. No. 2,745,297 to Andrus; U.S. Pat. No. 2,755,683 to Ryan; U.S. Pat. No. 3,251,243 to Kress; U.S. Pat. No. 3,503,279 to Sievert et al.; U.S. Pat. No. 3,861,485 to Busch; U.S. Pat. No. 4,599,916 to Hirosawa; U.S. Pat. No. 4,546,673 to Shigematsu; U.S. Pat. No. 4,644,820 to Macey et al.; U.S. Pat. No. 4,672,861 to Lanzer, U.S. Pat. No. 5,215,323 to Cowan; U.S. Pat. No. 3,944,253 to Ripley; U.S. Pat. No. 5,121,936 to Smirl; and U.S. Pat. No. 4,706,518 to Moroto. An epicyclical speed reduction mechanism has been described in U.S. Pat. No. 5,360,380 to Nottle, that purportedly produces variable output speed without using variable pulley belt drives or friction devices.
Several embodiments of the invention are described in this application. These are all capable of continuously varying the output speed and torque through forward, neutral, and reverse while the input shaft remains constant in rotational speed and direction. Each of the devices share: 1) the highly desirable features of compactness of both width and diameter; 2) use of efficient epicyclic gear components to achieve the infinitely variable gear-ratio feature; 3) integral features allowing for direct, independent speed control of the planetary elements driven from the input shaft; 4) coaxially aligned input and output shafts; and 5) a relatively small number of components for inexpensive, reliable manufacture.
The primary difference among the embodiments herein is in the means by which the independent speed control of the planetary gears is achieved, for which three principal means are presented. The first means is a serpentine belt or chain mechanism where the planetary gears are powered directly via belt pulleys or chain sprockets and an endless belt or chain is configured in a serpentine arrangement around three planetary drive pulleys and a central, rotationally fixed variable pitch pulley member. The chain drive permits significantly higher torque applications than the endless belt version, but is not continuously adjustable (rather is incrementally adjustable to integer values of the chain pitch). Central to the chain drive is a mechanical iris pulley member that can expand or contract to various diameters corresponding to the integer incremental values of the chain pitch, supporting an adjustment chain or beaded interface band in traction with the drive chain, and a mechanism for extension and retraction of the adjustment chain or beaded interface band. In this description and in the claims, a beaded interface band is functionally equivalent to a linked chain to serve as an adjustment chain.
The second means relies neither on belts nor chains for speed control, employing instead a set of tapered split races, coaxially aligned with the input and output shafts, that provide traction and position control to rollers mounted on swing arms coaxially aligned with each of the planetary gears and attached to the planetary carrier. Control means are provided to move the split tapered races relative to each other in axial extent which, by means of mating rollers and planetary moon gears meshed with each planetary, controls the speed of the planetary gears. The design allows for very high torques to be transferred to the output shaft throughout a wide range of speed variation by virtue of the addition of more swing arms and planetary elements providing for multiple points of engagement between rollers and races, as well as the positioning of the arms so that self locking between the rollers and races occurs as the output torque increases.
The third means combines the power from two prime movers of which one or both have speed adjustment capability. This dual input design allows for coaxial alignment of the input shaft from one of the prime movers with the output shaft, but requires the input from the second prime mover to be offset.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.